MICROWAVE CIRCUIT

Abstract: The invention relates to a microwave circuit comprising a two-sided substrate (1) with microwave components, a ground plane (5) and at least one interface means impedance-matched for external connection of the microwave circuit. The interface means has an external-connection point (6). The impedance matching of the interface means comprises a matching pattern (11) arranged on one side of the substrate (1) and connected to a ground plane (5) arranged on the opposite side. The matching pattern provides a circuit with excellent matching, thus giving low power losses at significantly higher frequencies than the prior art.

Description: MICROWAVE CIRCUIT The present invention relates to a microwave cir¬ cuit, more specifically a microwave circuit according to the preamble of claim 1. Microwaves are the designation of electrical signals in the frequency range 0.5-50 GHz. Circuits for handling microwaves, i.e. microwave circuits, generally consist of a substrate of aluminium oxide, or some other insulating material, on which conductors, resistors and optionally capacitors are built up by sputtering followed by the plating of patterns on the substrate. The other compo¬ nents are then soldered or glued on the substrate. The substrate is encapsulated in a capsule of conductive material. The circuit itself may be designed according to different principles. A basic design is described, for example, in US Patent Specification US-5,097, 233. The microwave circuit disclosed therein consists of a substrate on whose upper side components and connections are arranged. This cir- cuit has conductors and a ground plane in a single plane, a so-called coplanar structure. Another basic design is disclosed in US-4, 626, 805. The microwave circuit described therein is double-sided and consists of a substrate (26), on the upper side of which components are arranged and on the lower side of which a ground plane, coplanar conductors and connections are arranged. Through holes, being through-plated, are formed in the substrate for the connection of components directly to the ground plane and for electric connection of the connections to the inputs and outputs of the cir¬ cuit. In a circuit of this type, the lower side of the substrate is generally soldered on a lead frame, to the leads of which external components and circuits are con¬ nected. A cover is soldered to the upper side of the cir- cuit. In this manner, the circuit is hermetically enclos¬ ed and electrically shielded. When high-frequency signals, such as microwaves, pass an interface of some type, for instance between two circuits, it is important that the two channels meeting in the interface are electrically matched, i.e. have the same impedance (generally 50 ohm). If this is not the case, part of the signal is reflected, which means a power loss. Since the power at these frequencies is very expensive, especially in applications where a high output power is desired, great importance is attached to the matching. Losses due to poor matching increase with increasing frequency. Generally, it is quite difficult to obtain good matching. This applies perhaps in particular to the double-sided design described above (US-4,626, 805), to which the present invention is primarily directed. The path of a signal supplied to or from the circuit can be divided into three main parts, which may be said to form the above-mentioned interface, namely the connection lead, the through hole and the input or output conductor. The design of the main parts is decisive of the matching quality. The part which in the known microwave circuits has the greatest adverse effect on the matching, and hence determines the upper limit frequency of the circuits, is the input/output conductor. Predominant factors for the impedance of a conductor are its width and its distance from the ground plane. In the double-sided design, it is possible to control the impedance of the conductor with high precision, except in a portion adjacent the through hole. The reason for this is that a region around the through hole has no ground plane, which is self-evident unless it should be grounded. This absence of a ground plane entails a different impedance in the portion of the conductor which is situated on the upper side of the sub¬ strate opposite this region, and thus lacks a ground plane below it, than in the rest of the conductor. This portion gives an extra inductance. The through hole also provides a certain inductance, however considerably less since one has managed to solve the matching thereof quite well. The compensation for the inductance of the third part, i.e. the connection lead, is relatively easy to achieve by means of a gap having a suitable width between the lead and the ground plane. In circuits operating with small powers and at fre- quencies below about 8 GHz, the effect of the mismatch in the interface in most cases becomes acceptably small, but at higher frequencies and/or higher powers, the power loss becomes so noticeable that measures have to be taken to improve the matching. A certain improvement can be achieved by increasing the width of the conductor considerably in said portion, which lacks a ground plane below it. However, this does not solve the problem entirely and may also be a com¬ pletely impracticable way, for example when there is not sufficient room for the required widening. The above-mentioned US-4, 626, 805 shows another way of improving the matching. At the input on the lower side of the substrate, part of the ground plane is replaced by a coplanar conductor (30) , in one end (34) of which the through hole, or the via hole (40), is disposed and in the other end (32) of which the terminal for connecting another microwave circuit is located. The via hole con¬ nects the coplanar conductor to the input conductor (36) on the upper side of the substrate. A gap (6), surround- ing the end (34) of the coplanar conductor and the via hole, is provided. A capacitance which compensates for the inductances in the end of the input conductor and in the via hole, is then introduced, the width of the gap being dimensioned for obtaining the correct capacitance. This solution does not yield a satisfactory result. Simultaneously with the compensating capacitance, the gap gives rise to an inductance in the part of the input conductor located above the gap. The provision of a coplanar conductor on the lower side of the substrate entails that the via hole is positioned a considerable distance inside the edge of the substrate, and the input thus occupies a considerable, useless surface on the upper side of the substrate. US-4, 626, 805 also shows another embodiment, where an attempt has been made to compensate for the inductance obtained in the part of the input conductor which is located above the gap arranged on the lower side. A por¬ tion (T) of the input conductor is considerably widened, this giving a capacitive contribution. However, this com¬ pensation is not satisfactory because it is not made at the source of the mismatch. Nor is this type of compensa- tion feasible, since an increase of the capacitive con¬ nection by widening the conductor requires an underlying ground plane, which thus is not present where the mis¬ match occurs. The result in the form of circuit performance does not become acceptable by the measures mentioned above and described in US-4, 626, 805. Although the losses increase but slowly up to 20 GHz, this is a breakpoint, above which they increase very rapidly. Therefore, the circuit is practically unusable for frequencies above 20 GHz. One object of the present invention is to provide a microwave circuit with improved matching in the interface of the circuit as compared with known microwave circuits. Another object of the invention is to provide a microwave circuit having small power losses at frequen- cies up to above 26 GHz. These objects are achieved by means of a microwave circuit of the type stated in the introductory part and having the features recited in the characterising clause of claim 1 of the appended claims. This solution of the matching problem is applicable to microwave circuits of practically any design and inde¬ pendently of the component content and component posi- tions, design principle etc. The provision of the match¬ ing pattern on the opposite side of the substrate with respect to a ground plane means that the mismatch can be dealt with at the source, as opposed to prior-art solutions. Moreover, the matching pattern has favourable effects only and poses no problems, as do the solutions according to US-4, 626, 805. The matching pattern according to the present invention thus makes it possible to attain low losses at very high frequencies by suitably designing the matching pattern. Embodiments of a microwave circuit according to the present invention will be described in more detail below with reference to the accompanying drawings, in which: Fig. 1 shows a part of a substrate with a matching pattern according to the invention; Fig. 2 shows the matching pattern in detail; and Fig. 3 shows a portion of the lower side of the sub¬ strate. Fig. 1 shows a cut-away, enlarged portion of a sub- strate 1. In practice, the substrate 1 has a length of about 15-20 mm and a width of about 10-15 mm. The sub¬ strate 1 is arranged on a lead frame and is covered by a cover which hermetically encloses the substrate 1 and also produces a shielding effect. The lead frame and the cover are not shown, since they are of secondary impor¬ tance for the invention. On the upper side 2 of the sub¬ strate 1, microwave components and conductor patterns 3 are arranged (most of which are not illustrated for pur¬ poses of clarity), and on the lower side 4 of the sub- strate are provided a ground plane 5 and connections 6. Lead-throughs in the form of through holes 7, so-called via holes, connect the upper side 2 and the lower side 4 to each other. The via holes 7 are through-plated or completely filled with conductive material. The via holes 7 are used for transmitting signals between the connec¬ tions 6 and conductors 3 or components, and for directly grounding the components on the upper side etc. The ground plane 5 covers the major part of the lower side 4, but leaves a gap 8 with bare substrate material around each connection 6 that is not to be grounded. On the upper side 2 are provided (see Fig. 1 ) a grounded frame 9, to which the cover is connected by soldering, an input (or an output) conductor 10 and an embodiment of a match¬ ing pattern 11 according to the present invention. These components are also shown in Fig. 2, which is a cut-away portion (further enlarged in relation to Fig. 1) of the substrate 1 as-seen from above. Fig. 3 shows a cut-away portion of the substrate 1 as seen from below. Fig. 3 is on the same scale as Fig. 1. As mentioned above, it is essential that the micro¬ wave circuit is well matched, i.e. that the impedance in the inputs and outputs of the circuit is the same as in the connected, external conductors. A common impedance is 50 ohm. If the impedance is not equal, signal reflections will result in the junction, which means power losses. In the illustrated embodiment of a microwave circuit according to the invention, the critical junction is the connection 6, the via hole 7 and a part of the input con¬ ductor 10, which will be explained in more detail hereinbelow. In this embodiment, the input conductor 10 extends at right angles to an end edge 12 of the substrate 1, from the via hole 7 and inwards to an input component of the circuit. The factors which, in the type of circuit design here shown, have the greatest impact on the impe¬ dance of a conductor are the distance of the conductor to the ground plane and the width of the conductor. Since substantially the entire lower side 4 of the substrate 1 is covered by the ground plane and the thickness of the substrate is well-defined, it is relatively easy to dimension the input conductor 10 so that its impedance becomes 50 ohm. There is however one important exception to this. An end portion 14 of the input conductor 10 and a pattern portion 13, which is in the form of a rectangular collar surrounding the opening of the via hole 7 and being wider than the input conductor 10, have no ground plane straight underneath them because of the compulsory gap 8, see especially Fig. 2. The collar 13 is provided when plating the via hole 7. This results in a mismatch which primarily has a substantial adverse effect at fre¬ quencies above about 8 GHz, the power losses increasing heavily with increasing frequency. In the end portion 14, the input conductor 10 constitutes an extra inductance giving an excessive total impedance. To this come induc¬ tances caused by the collar 13, in the via hole 7 and in the connection 6. The whole of this incompletely matched part, i.e. the connection 6, the via hole 7, the collar 13 and the end portion 14, will be referred to herein¬ after as interface. The inductance in the connection 6 is compensated for according to known technique, by dimen¬ sioning the gap 8 for suitable capacitive connection to the ground plane 5. According to the invention, the problem of mismatch in the rest of the interface is solved practically completely by the provision of the matching pattern 11. The matching pattern introduces a capacitive counterbalance to the inductances in the entire interface even if it primarily eliminates the mismatch in the end portion 14 and the collar 13. The matching pattern 11 is connected to the ground plane 5 through via holes 15 and may thus be considered as an extra ground plane placed on the upper side 2. The matching pattern 11 consists of two parts which are sym¬ metrically arranged on both sides of the input conductor 10 and which are stepped, i.e. they have a stepped bound¬ ary line with two steps facing the input conductor 10. Study the lowermost portion of the matching pattern in Fig. 2. What is said below about this portion also simi¬ larly applies, because of the symmetry, to the other por¬ tion. The first and second steps 16 and 17, respectively, of the stepped part are parallel to the input conductor 10 and differently spaced from the input conductor 10 so as to form, respectively, a first and a second gap 18 and 19, respectively. A first connecting edge 20, which is perpendicular to one of the steps 16, 17, connects these with each other. The first step 16 extends along the end portion 14 from the rim of the opposite gap 8 towards the pattern portion 13 and leaves only a portion immediately adjacent the pattern portion 13 where the second step 17 starts. The second step 17 extends up to an imaginary straight line which is perpendicular to the step 17 and passes through the centre of the via hole 7. A second connecting edge 21, which is parallel to the first con¬ necting edge 20, connects the second step 21 to a "floor" 22 which extends up to the frame 9. This matching pattern 11 thus introduces a capaci¬ tive counterbalance to the above-mentioned inductance in the end portion 14 and the pattern portion 13 by intro¬ ducing a capactive connection to the ground plane. Alter- natively, the matching pattern 11 can be designed by minor changes so as to also compensate for the inductance in the via hole 7. If so desirable, the matching pattern 11 may also be designed so as to compensate for part of the inductance/capacitance of the connection 6, the gap 8 being adjusted to a corresponding extent. The width of the gaps is essential to the impedance in the region. In this embodiment of the microwave cir¬ cuit according to the invention, the width of the input conductor is 0.24 mm; the width of the first gap 18 is 0.1 mm and the width of the second gap 19 is 0.3 mm. Of importance also is the distance between the first con¬ necting edge 20 and the pattern portion 13, as measured at right angles to the first connecting edge 20. In this embodiment, this distance is 0.1 mm. The via hole 7 has a diameter of 0.25 mm and its centre is located 0.25 mm from the nearest side edge of the end portion 14. The length of the first step 16 is 0.45 mm, and the length of the second step 17 is 0.35 mm. The distance between the end portion 14 and the nearest lead-through 15 is 1.0 mm in this microwave cir- cuit, which is intended for use at about 10 GHz. This distance may be arbitrarily short but should not exceed a 1/8 wavelength (i.e. λ/8). In this example, it is suit¬ able to maintain the distance below about 2.0 mm. In the frequency range where the microwave circuit according to the invention is intended to operate, the upper limit of this distance is about 3.0 mm in practice. The majority of the measures indicated above depend on the thickness of the substrate 1 and many also depend on the width of the conductor 10. The thickness of the substrate 2 generally is 0.25 mm, 0.38 mm or 0.64 mm, and in this case it is 0.25 mm. This thickness determines the width of the conductor 10, which in turn primarily deter¬ mines the width of the gap 18. Depending on the thickness of the substrate 1, the width of the conductor 10 and the length of the step 16 (step 16 may extend along a minor part of the conductor 10 with a simultaneous decrease of the width of the gap 18), the width of the gap 18 may vary between almost 0 and about 0.5 mm, the lower limit being essentially determined by what is feasible in terms of manufacturing technique (today about 0.01 mm). The embodiment of the microwave circuit according to the invention as described above is of course not the only possible solution, but modifications can be made as long as they fall within the scope of the invention such as it is defined in the claims. For example, the design of the matching pattern can be changed according to the conditions concerning the design of the conductor, the thickness of the substrate, the extent and the shape of the ground-planeless portion on the lower side of the substrate and so forth. Thus, the matching pattern need not be connected to the frame. The number of via holes in the matching pat- tern may vary. As mentioned above, the matching pattern can be designed so as to give a higher capacitance than in the embodiment described above to compensate for the inductance in the via hole. On the other hand, the match- ing pattern can, if so desired, be designed for example so as not to compensate for the mismatch in the pattern portion around the via hole. This however yields a poorer result at higher frequencies, but may nevertheless confer certain advantages. Nor is the symmetry which the match- ing pattern exhibits in the illustrated embodiment neces¬ sary. Also, the stepped shape of the matching pattern should be considered as an example only. The periphery of the matching pattern may of course be given any other shape, provided the result is that intended by the inven- tion.

Claims: CLAIMS 1. A microwave circuit comprising a two-sided sub- strate (1) with microwave components, a ground plane (5) and at least one interface means impedance-matched for external connection of the microwave circuit and compris¬ ing an external-connection point (6), c h a r a c t e r ¬ i s e in that the impedance matching of the interface means comprises a matching pattern (11) arranged on one side of the substrate (1) and connected to a ground plane (5) arranged on the opposite side. 2. A microwave circuit as claimed in claim 1, in which the interface means further comprises a conductor (10) and a lead-through (7) connecting the conductor (10) to the external-connection point (6), a portion (14) of the conductor (10) extending over a ground-planeless area (8) arranged on the opposite side of the substrate, c h a r a c t e r i s e d in that the matching portion (11) is disposed at least at said portion (14) of the conductor (10) and, in the first place, compensates for the absence of a ground plane (5) in the ground-planeless area (8) . 3. A microwave circuit as claimed in claim 2, c h a r a c t e r i s e d in that the ground-planeless area (8) consists of an interspace (8) separating the external-connection point (6) from the ground plane (5). 4. A microwave circuit as claimed in any one of claims 2 and 3, c h a r a c t e r i s e d in that there exists a gap (18) between the matching pattern (11) and said portion (14), said gap (18) having a width in the range of from above 0 to 0.5 mm. 5. A microwave circuit as claimed in any one of claims 2-4, c h a r a c t e r i s e d in that the match- ing pattern (11) further is provided at a pattern portion (13) surrounding the lead-through included the interface means, and that there exists a gap (19) between the matching pattern (11) and the pattern portion (13), said gap (19) having a width in the range of from above 0 to 0.5 mm. 6. A microwave circuit as claimed in any one of claims 2-5, c h a r a c t e r i s e d in that the match¬ ing pattern (11) consists of two parts arranged on each side of said portion (14). 7. A microwave circuit as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the matching pattern ( 11 ) is connected to the ground plane (5) via one or more lead-throughs (15). 8. A microwave circuit as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the matching pattern (11) is connected to the ground plane (5) via several lead-throughs (15) comprising through- plated through holes, or completely or partly filled via holes, or combinations thereof. 9. A microwave circuit as claimed in any one of claims 7-8, c h a r a c t e r i s e d in that the dis- tance between the portion (14) and the most adjacent lead-through (15) in the matching pattern is £3.0 mm. 10. A microwave circuit as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the matching pattern (11) is connected to a frame (9) of con- ductive material arranged on the substrate (1).

Inventors: PERSSON LARS, PERSSON, LARS
Applicants: PERSSON LARS, SIVERS IMA AB, PERSSON, LARS
Application year: 1994
Main classification: H01P3
IPC codes: H01P5, H05K1, H05K3